As a general matter, prior art emulsion polymerizations utilize a chain growth mechanism, wherein an initiator adds to a monomer to form a reactive end group which then reacts with another monomer molecule. Additional monomer units are added to the reactive end group until some termination reaction takes place wherein the reactive end group is quenched. One or more monomers and one or more surfactants are dispersed in an aqueous medium. Sufficient levels of surfactants are employed to reach a critical micelle concentration (CMC).
Prior art emulsion polymerization methods are generally not suitable for step growth polymerizations using one or more condensation monomers. By “condensation monomer,” Applicants mean a first monomer which reacts either with another first monomer, or with a second monomer, to liberate water as a reaction product. Step growth polymerizations include formation of a polyester using a mixture of a di-acid monomer and a di-ol monomer, or a monomer comprising both an acid group and an alcoholic group. Similarly, polyamides can be formed using a mixture of a di-acid monomer and a di-amine monomer, or a single monomer comprising an acid group and an amino group, i.e. an amino acid.
In such a step growth polymerizations, an acid group on a first monomer reacts with, for example, an alcohol/amine group disposed on a second monomer to liberate water and form an ester/amide linkage, respectively, interconnecting the first monomer with the second monomer. The process is repeated, and the molecular weight of the reaction product increases.
As a further general matter, prior art emulsion polymerization methods are virtually always initiated using a free radical. Anionic or cationic reactive chain ends would be rapidly quenched by the aqueous solvent.
Using prior art emulsion polymerization methods, the interior of each micelle provides the site necessary for polymerization. A monomer, such as for example styrene or methyl methacrylate, and a water soluble free radical initiator are added and the reaction mixture is agitated. The product of such an emulsion polymerization is sometimes referred as a “latex.”
In the reaction mixture, the monomer(s) can be found in three different places. Those one or more monomers may be disposed in large monomer droplets disposed in the aqueous solvent. Some of the monomer, albeit very little, may be dissolved in the water. Lastly, the one or more monomers may be found in micelles.
Initiation takes place when an initiator fragment migrates into a micelle and reacts with a monomer molecule. Water soluble initiators, such as peroxides and persulfates, are commonly used to, inter alia, prevent polymerization in the big monomer droplets. Once polymerization starts, the micelle is referred to as a particle. Polymer particles can grow to extremely high molecular weights, especially if the initiator concentration is low. That makes the radical concentration and the rate of termination low as well. Sometimes a chain transfer agent is added to the mix to keep the molecular weight from getting too high.
Monomer migrates from the large monomer droplets to the micelles to sustain polymerization. On average, there is one radical per micelle. Because of this, there isn't much competition for monomer between the growing chains in the particles, so they grow to nearly identical molecular weights and the polydispersity is very close to one. Practically all the monomer is consumed in emulsion polymerizations, meaning the latex can be used without purification. This is important for paints and coatings.
Each micelle can be considered as a mini bulk polymerization. Unlike traditional bulk polymerizations there is no unreacted monomer leftover, and no thermal “hot spots” form. In bulk polymerizations, thermal hot spots cause degradation and discoloration and chain transfer broadens the molecular weight distribution.
What is needed is a method to polymerize one or more condensation monomers, where those condensation monomers are dispersed in a non-aqueous solvent system, and where those one or more condensation monomers are essentially insoluble in that non-aqueous solvent system.